National Park Service U.S. Department of the Interior Natural Resource Stewardship and Science

Salt Marsh Elevation Monitoring at Assateague Island National Seashore: 2009 - 2024

Figure 1: Surface elevation table (SET) monitoring at Assateague Island National Seashore.

Background

Coastal wetlands exist within a narrow range of elevation which is influenced by local hydrologic conditions. As sea level rises, coastal wetland systems must build elevation at a rate equal to or greater than the rate of sea-level rise to maintain favorable hydrologic conditions for their survival (Figure 2). With global rates of sea-level rise predicted to increase over the coming decades (Church et al. 2010), many more coastal wetlands may become vulnerable to increased inundation. Additionally, certain areas of the coast, such as the mid-Atlantic region, can undergo even greater sea-level rise due to changes in coastal ocean circulation (Sallenger et al. 2012).

The Northeast Coastal & Barrier Inventory and Monitoring Network (NCBN) uses the SET-MH technique (surface elevation table-marker horizon) to monitor elevation change as a vital sign of salt marsh health at Assateague Island National Seashore. In the following sections, we use these data to answer the following questions:

1. What are the elevation characteristics of coastal wetlands in Assateague Island National Seashore? (Figure 2A; Results Section 1)

2. How do local wetland elevation dynamics relate to the local hydrology (e.g., local tidal datums)? (Figure 2C; Results Section 2)

3. Is elevation gain sufficient to keep pace with current and predicted sea level rise? (Figure 2D; Results Section 3)

Figure 2: Conceptual diagram showing the relationship among wetland surface elevation (A), tidal datums (C), and their respective rates of change (B and D). Understanding these relationships is important for evaluating the sustainability of a wetland community to sea-level rise.

Methods

Protocols (Vital Signs)

NCBN’s long-term, standardized monitoring protocol for monitoring surface elevation change (Lynch et al. 2015) is implemented semiannually in the spring and fall at Assateague Island National Seashore. Changes in surface elevation are measured using the SET-MH technique (surface elevation table (SET) – marker horizon (MH)):

• The SET is a portable mechanical leveling device providing repeated, high-resolution measurements of elevation change in wetland sediments or shallow water bottoms relative to the depth of a permanent benchmark that has been anchored into the soil until refusal. During measurements, the SET arm is attached to the permanent SET benchmark and extended over the marsh surface at four fixed positions. The SET arm is carefully leveled to rest horizontally to the ground, and each of nine fiberglass pins are lowered through the arm to the soil surface. The height of each pin above the arm is measured on repeated sampling events. Changes in the height of the pins between sampling events are used to quantify soil surface elevation change over time relative to the permanent benchmark.

• Marker horizons are artificial soil layers (e.g., feldspar) established on the surface of wetland or shallow water bottoms to measure subsequent surface sediment accretion. Cores are taken with a soil corer from the soil surface to this layer on repeated sampling events, and the thickness of the sediment accumulated above the layer is measured as vertical accretion.

Sample Size

Surface elevation change is actively monitored at four sites within Assateague Island National Seashore (Figure 3). A fifth site, Pine Tree Study, was monitored between 2016 to 2022 as part of a pilot effort to compare rates of surface elevation change derived from SETs to rates derived by other elevation measurement methods (Lynch et al. 2024). Additional details on the status of each site, the number of replicate SET-MH stations at each site, and the dates of the initial and most recent measurements are shown in Table 1 below.

All data utilized in this report are available for public use and can be downloaded from the NPS DataStore.

Table 1: Surface elevation table sites monitored by NCBN within Assateague Island National Seashore.

Site	SET-MH station count	First reading	Most recent reading	Status
Marsh 11	4	2009	2024	active
Marsh 5 (Pope Bay)	4	2009	2024	active
Marsh 6 (Pine Tree)	4	2009	2024	active
Marsh 8 (Valentines)	4	2009	2024	active
Pine Tree Study	3	2016	2022	inactive

Figure 3: Individual SET stations within Assateague Island National Seashore. Click on the points to see the site and station labels.

Results

1. What are the elevation characteristics of coastal wetlands at Assateague Island National Seashore?

Across all four actively monitored sites at Assateague Island National Seashore, the park-wide average rate of surface elevation change was 4.14 ± 0.72 mm/yr. Site-level rates of surface elevation change varied from a low of 2.86 ± 0.23 mm/yr at Marsh 5 (Pope Bay) to a high of 5.51 ± 0.19 mm/yr at Marsh 8 (Valentines) (Table ??; Figure 4).

Figure 4: Site-level surface elevation change at Assateague Island National Seashore. Rates of surface elevation change (SEC) are shown in panel labels. Hover the mouse over a data point to see the cumulative surface elevation change value on a specific date, or click and drag over a specific time to zoom in. Double click to zoom back out

2. How do local wetland elevation dynamics relate to the local hydrology (e.g., local tidal datums)?

3. Is elevation gain sufficient to keep pace with current and predicted sea-level rise?

Comparisons to current sea-level rise

As of August 2025, the current long-term rate of relative sea-level rise from the nearest NOAA tide gauge at Ocean City, MD was 5.28 mm/yr (± 0.69) for the period between 1975 to 2024 (https://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?id=8570283). Out of the five sites at Assateague Island National Seashore, two sites - Marsh 6 (Pine Tree) and Marsh 8 (Valentines) - were gaining elevation at a rate close to or greater than the relative long-term rate of sea-level rise.

Additionally, using the sea-level data from the nearby NOAA tide gauge, we calculated a recent rate of relative sea-level rise of 7.26 mm/yr (± 0.80) for Assateague Island National Seashore over the most recent tidal epoch (2001-2019). None of the sites at Assateague Island National Seashore were building elevation at a rate close to or greater than the relative recent rate of sea-level rise.

Comparisons to future sea-level rise

Based on the calculations provided in Sweet et al. 2022, future predicted rates of relative sea-level rise by 2100 for Assateague Island National Seashore ranged from a low of 6.00 mm/yr for the “low” sea-level rise scenario to a high of 38.00 mm/yr for the “high” sea-level rise scenario (Table ??2).

Table 2: Predicted future sea-level rise by 2100 for Assateague Island National Seashore. Future predicted sea-level and rates of sea-level rise were based on calculations for the nearest NOAA tide gauge provided in Sweet et al. 2022 for the 5th National Climate Assessment .

Park	NOAA gauge ID	Sea level rise scenario	Sea level rise by 2100 (m)	Future predicted SLR rate by 2100 (mm/yr)
ASIS	8570280	Low	0.3	6.00
		Intermdiate-Low	0.5	9.00
		Intermediate	1.0	22.00
		Intermediate-High	1.5	29.00
		High	2.0	38.00

References

Lynch, J.C., P. Hensel, and D.R. Cahoon. 2015. The surface elevation table and marker horizon technique: A protocol for monitoring wetland elevation dynamics. Natural Resource Report NPS/NCBN/NRR—2015/1078. National Park Service, Fort Collins, Colorado.

Church, J.A, P.L. Woodworth, T. Aarup, and W.S. Wilson. 2010. Understanding sea level rise and variability. Blackwell, New York, NY.

Lynch, J.C., N. Winn, K. Kovalenko, and G. Guntenspergen. 2024. Comparing wetland elevation change using a surface elevation table, digital level, and total station. Estuaries and Coasts, 47, 2071-2079.

Sweet, W.V., B.D. Hamlington, R.E. Kopp, C.P. Weaver, P.L. Barnard, D. Bekaert, W. Brooks, M. Craghan, G. Dusek, T. Frederikse, G. Garner, A.S. Genz, J.P. Krasting, E. Larour, D. Marcy, J.J. Marra, J. Obeysekera, M. Osler, M. Pendleton, D. Roman, L. Schmied, W. Veatch, K.D. White, and C. Zuzak. 2022. Global and Regional Sea Level Rise Scenarios for the United States: Updated Mean Projections and Extreme Water Level Probabilities Along U.S. Coastlines. NOAA Technical Report NOS 01. National Oceanic and Atmospheric Administration, National Ocean Service, Silver Spring, MD, 111 pp.

Ainsworth, A., Berkowitz, P., Jacobi, J.D., Loh, R.K., & Kozar, K. 2011. Focal Terrestrial Plant Communities Monitoring Protocol: Pacific Island Network. Natural Resource Report NPS/PACN/NRR—2011/410. National Park Service, Fort Collins, Colorado.

Contact Information

Laura Feher, Biologist
Laura_Feher@nps.gov

Vital Signs: Northeastern Coastal & Barrier Inventory and Monitoring Network
Reports & Publications (U.S. National Park Service) (nps.gov)